Our studies on mechanisms of cell death after photodynamic therapy (PDT) have suggested a novel procedure for enhancing phototoxic efficacy. We had previously shown that photodynamic damage to mitochondria catalyzes release of cytochrome c into the cytosol, triggering an apoptotic response. This can bypass defective steps in the apoptotic program elicited by conventional chemotherapy. Several recent reports have indicated that the bile acid ursodeoxycholic acid (UDCA) could protect the mitochondrial membrane of hepatoma cells from the pro-apoptotic effects of several agents including ethanol, deoxycholic acid, hydrogen peroxide and cadmium ion. We expected that UDCA would also protect mitochondria, an important PDT target, from the induction of an apoptotic response after PDT. We found instead the opposite effect: a significant promotion of cytochrome c release, caspase-3 activation and apoptotic cell death. This was demonstrated in both murine leukemia and hepatoma lclc7 cells. In the former cell line, we also demonstrated that UDCA could promote the apoptotic response to photosensitizing agents that catalyze selective photodamage to the anti-apoptotic protein Bcl-2, but do not affect the pro-apoptotic protein Bax. In a pilot study, we found the lifespan of mice bearing the RIF tumor was enhanced when UDCA was administered before irradiation, using the tin etiopurpurin SnET2 as the photosensitizing agent. These results are of potentially great clinical significance since UDCA is widely used for the treatment of gallstones and liver diseases, and its pharmacology and safety have been well established. To provide information on the pertinent mechanisms, we plan to [1] characterize the effects of UDCA on the cytotoxic responses to a spectrum of photosensitizing agents, [2] assess the structure-activity relationships in a series of UDCA analogs including the glycine- and taurine-conjugates that are formed in vivo, [3] examine the efficacy of UDCA for PDT enhancement in a variety of tumor cell lines, [4] measure PDT-promotion by UDCA and selected analogs in animal tumor models. Our hypothesis is that UDCA partitions into mitochondrial membranes and both lowers the threshold of photodamage, while offering protection from the chaotropic effects of more hydrophobic bile acids. Aim [4] will be carried out in a sub-contract with the PDT facility at University of Louisville where there is the required expertise in animal PDT studies. If the results of preliminary studies are borne out by further investigation, UDCA could offer a safe and effective means for promoting clinical PDT efficacy.